1. Field of the Invention
The present invention relates to active-matrix liquid crystal display devices and methods for driving the devices, and in particular to an activation control technique used when power is supplied.
2. Description of the Related Art
FIG. 9 shows a general block diagram of a conventional liquid crystal display device. This liquid crystal display device includes a video driver 101, a timing generator 102, a voltage regulation circuit 103, and a liquid crystal panel 104. The video driver 101 performs synchronous separation and decoding by processing a video signal "VIDEO" inputted from the exterior. A synchronizing signal "SYNC" obtained by synchronous separation is sent to the timing generator 102. The timing generator 102 reversely supplies a field reverse pulse signal "FRP" to the video driver 101. The video driver 101, which includes a driver unit, converts a video signal demodulated by decoding, into ac video signals "V.sub.sig " for driving liquid crystal in accordance with the field reverse pulse signal FRP. These ac video signals V.sub.sig consist of three primary color components: red, green and blue, which are outputted. The timing generator 102 generates various timing signals in accordance with the synchronizing signal SYNC in addition to the above field reverse pulse signal FRP. The timing signals include horizontal start-pulse signal "HST", horizontal clock signals "HCK1" and "HCK2", vertical start-pulse signal "VST", and vertical clock signals "VCK1" and "VCK2", which are all supplied to the liquid crystal panel 104. The voltage regulation circuit 103 supplies counter voltage "V.sub.com " to the liquid crystal panel 104. The liquid crystal panel is provided with a counter electrode and pixel electrodes both in contact with a liquid crystal layer. The counter voltage V.sub.com is applied to the counter electrode, while the video signals V.sub.sig are applied to the pixel electrodes. The liquid crystal panel 104 is provided with liquid crystal pixels arranged in a matrix between the counter electrode and the pixel electrodes. Since the liquid crystal panel 104 is a built-in peripheral driving circuit type, it includes a vertical scanner and a horizontal scanner. The vertical scanner, which operates in accordance with the vertical start-pulse signal VST, sequentially selects each row of the liquid crystal pixels. The horizontal scanner, which operates in accordance with the horizontal start-pulse signal HST, writes the video signals V.sub.sig to the selected row of the liquid crystal pixels by sequentially distributing them to each column of the liquid crystal pixels.
As described above, the liquid crystal display device is driven by applying the video signals V.sub.sig, the counter voltage V.sub.com, and various timing signals including the horizontal start-pulse signal HST and the vertical start-pulse signal VST to the liquid crystal panel 104. Predetermined power voltages are also supplied to the horizontal scanner and the vertical scanner which are built into the liquid crystal panel 104. According to a conventional device, the video driver 101, the timing generator 102 and the voltage regulation circuit 103 are activated at the same time when the power is supplied. However, when the power is supplied, the way in which the video signals, the timing signals and the counter voltage rise is not regular due to the characteristics of integrated circuits included in the video driver 101, the timing generator 102 and the voltage regulation circuit 103. After the video driver 101, the timing generator 102 and the voltage regulation circuit 103 have been activated, the video signals V.sub.sig and the counter voltage V.sub.com reach their stable conditions through their transition conditions. These changes are shown on a graph in FIG. 10. On the graph the horizontal axis represents time t with one graduation set to 20 ms, and the vertical axis represents voltage with one graduation set to 5 V. As can be seen, after the supply of power, the counter voltage V.sub.com gradually rises from the ground level (GND) to reach its normal level (for example, in proximity to 6 V). The video driver 101 outputs a dc voltage exceeding 10 V until its operation becomes stable after it has been activated. After that, the output is switched to the predetermined video signal. The graph shows that, after the supply of power, the output voltage of the video driver 101 rises to a dc voltage level exceeding 10 V from the ground level GND, relatively faster than the counter voltage V.sub.com. In other words, a relative delay is generated while both are rising. The delay is in the order of, for example, 10 to 100 ms.
FIG. 11 shows a graph of the potential difference (V.sub.com -V.sub.sig) between the counter electrode potential and the pixel electrode potential, on which an effective driving voltage applied to the liquid crystal pixels is shown. On the graph the horizontal axis represents time t with one graduation set to 50 ms, and the vertical axis represents voltage with one graduation set to 5 V. In an initial phase after starting the supply of power, the effective voltage falls in proximity to -10 V, which causes application of an excessive dc component. This excessive dc component corresponds to the relative delay in the rise of the signal voltage V.sub.sig with respect to the counter voltage V.sub.com as shown on the graph in FIG. 10. In other words, the signal voltage rises to a dc level exceeding 10 V before the potential of the counter electrode reaches the vicinity of 6 V as its normal level, thus, the excessive dc component is transitionally applied to the liquid crystal pixels. Subsequently, the effective driving voltage applied to the liquid crystal pixels shifts to a stable condition through an unstable condition. In the stable condition an ac signal applied to the liquid crystal pixels includes a dc component. Transition from the start of the supply of power to the stable condition requires a period of 10 to 200 ms. The dc voltage is applied to the liquid crystal pixels in this manner until operation of the video driver for generating the video signals becomes stable after starting the power supply. The application of the dc voltage causes temporary irregularity in the orientation of the liquid crystal, which results in such significant display deterioration as to show luminescent spot defects over the screen. Such irregularity in the orientation of the liquid crystal may remain even after the dc component has been removed. According to the conventional art, the above-described problem occurs whenever the liquid crystal display device is activated, which causes not only display deterioration but also reliability deterioration. Accordingly, one of the problems to be solved is to improve reliability. The screen looks as if it has luminescent spot defects over its entire area for a period of time after starting the supply of power. Thus, in this unstable condition, measures to control the screen so the spots are seen by de-activating a backlight are taken. However, the measures are not effective in maintaining reliability because they are not fundamental and cannot prevent the dc component from being applied to the liquid crystal.